Vehicle brake systems, which are provided with a servo unit for boosting the braking effort when a brake pedal is pedaled, are widely known. For example, Patent Document 1 discloses an electric brake actuator (electric servo unit) using an electric motor as a force boosting source.
The electric brake actuator disclosed by Patent Document 1 is configured as follows. A main piston is formed by a shaft member that moves forward and backward by an operation of a brake pedal, and a booster piston is formed by a cylindrical member outer fitted to the shaft member (main piston). The electric brake actuator is arranged such that a thrust force that is input from the brake pedal to the shaft member (main piton) and a thrust force applied by an electric motor to the cylindrical member (booster piston) generate a hydraulic brake pressure to boost the braking effort.